yes those temperatures, especially load ones are way too high. Typically idling should be around high 30s to low 40s.
Full load, low-mid 80s, anymore than that and you're bordering on thermal damage. (Have you had any crashes/BSODs? if yes, you probably already having overheating issues)
My suggestions to deal with heat:
1. this is probably the most neglected
BLOW OUT THE DUST - get a can of compressed air from a store for like 5-10 bucks, turn the computer off and giving a good blow thru. If you not afraid to open in up, that'd be helpful too, you'll get more dust out.
2. When using a laptop, make sure you aren't blocking any air vents, make sure that your room temperature is not mid 80s and if none of that helps, go to power management and turn down some things that are listed under power saving ( like LCD brightness etc) the less power your pc uses the cooler it can stay.
3. get a laptop cooler, but since you already got one, you might want to revise how well its working. Not to bash it, but from the look of it there's 2 tiny fans that suck the air from the sides and bottom to push it through a huge cross sectional area that is not directly on top of the fans. Plus those are axial fans as far as I can tell. Basically the problem with those they are noisy and they don't deal very well with increased resistance, which is what you'd be seeing when you put a laptop on top of it. To tell you the truth no HVAC engineer would be impressed with that cooler.
but make sure in your cooler usage you aren't blocking its air intakes, like the huge hole it has on the bottom, that seems to be the primary intake. All the cooler will do is help circulate the air, if the air you're trying to circulate is hot, then it doesn't help any. Also, clean the dust from the cooler, typically they have less places to accumulate it but it still builds up overtime.
To AntiZig first:
1. I do this on a regular basis (every week) so I don't think that dust accumulation is the issue. I never use the laptop in a dusty environment either.
2. None of the vents are blocked, but I was thinking about the power usage of the cpu/gpu. I was thinking of undervolting, but I have never done it before. I have read up on the method to do it, but I'm wary of the risks of doing it.
3. The cooler is definitely not working as well as in the winter. The ambient temperature in the room is around 25 Celsius. In the winter, I keep it closer to 20. In addition, because the Zalman has an aluminum frame, it is very susceptible to these temperature changes. During the winter, the cooler is extremely cold to the touch and thus blows very cold air into the computer. Right now, it's kind of lukewarm. It certainly helps a lot more during the winter because of this. However, other coolers I've looked into with higher fan speeds are either made of plastic or don't fit well with the vents.
I was thinking about changing the thermal paste to AS5, but these Dell laptops seem like they were not made to be opened up easily. I understand that Dell has a service manual that provides steps to open it up, but it seems to involve a lot prying soft components...
Is there a video guide to disassembly of the XPS 15?